1. Field of the Invention
The invention relates to a method for drying stacked wood with the help of a drying gas guided in a circulatory flow, with the stacks of wood being charged zone by zone depending on the average moisture of the wood in the respective zone with partial streams of the drying gas which differ with respect to their drying power, and to an apparatus for performing the method.
2. Description of the Prior Art
Sawn timber is dried to a large part in wood drying systems to a desired final moisture. It is always a goal, after the completion of the drying process, to obtain the most uniform final moisture of the batch of wood. A batch of sawn timber is introduced into a treatment chamber for drying purposes and then dried at predetermined drying temperatures and at high air humidities. In the interior of the treatment chamber a circulatory flow for heated drying gas through the sawn timber is produced with fans. In order to ensure an even temperature distribution in the treatment chamber it is necessary to introduce any ambient air that may have been introduced into the treatment chamber as evenly as possible over the cross section of the treatment chamber, which on the other hand is a necessary precondition for an even distribution of the air moisture within the chamber. Since temperature differences in the treatment chamber lead to uneven final humidities it is known to divide the treatment chambers into several zones in order to regulate in these zones the chamber temperature separately per se to the common setpoint value applicable for the entire treatment chamber. Every zone comprises its own temperature sensors and has its own actuators for associated heating registers or groups of heating registers through which the temperature of the drying gas for each zone is regulated. Usually, the regulation of the heating register temperature occurs in such a way that the drying gas circulated in the treatment chamber and emerging from the stack is held at a constant temperature, as a result of which an evening of the transmitted thermal output occurs between the individual zones. The evaporation of the moisture supplied by the wood leads in each zone to a temperature drop which is inversely proportional to the circulated air quantity. It may occur with this type of regulation that the inlet temperature exceeds a permissible limit temperature and thus damage to the wood (formation of cracks, etc.) occurs.
Known apparatuses for drying stacked timber in a drying gas (DE 297 23 003 U1, DE 37 38 806 A1) comprise, among other things, a drying chamber, fans for circulating the drying gas and heating registers for heating the drying gas. For exchanging the drying gas in the heating chamber it is provided with a feed and discharge line for the drying gas, with valves being provided in the feed and discharge lines for regulating the exchange routes. These known apparatuses comprise only one zone however, so that they can hardly be used to achieve even wood humidities after a drying process when the treatment chamber is charged with wood batches of different moisture.
The goal in the known drying methods is always providing the drying power in a treatment chamber as evenly as possible over the entire cross section. As a result of the even drying power in the individual zones it is always noticed that the final moisture is not within the desired narrow margin. Such differences in final moisture are especially clear when the wood is dried so as to achieve relatively high final moisture values. This leads to the consequence that a batch of wood simultaneously contains considerable shares of wood that is insufficiently dried and wood that is too humid, which leads to a considerable share of rejects.
In the case of an even distribution of the mean initial humidities per zone, a favorable, even final moisture of the wood batch is achieved with the known method. However, if the initial moisture distribution of the individual zones is different, this distribution will have a direct influence on the final result due to the desired even drying power. Differences in initial moisture can be felt in particular which are present in the longitudinal direction of the chamber because there is no considerable exchange of air transversally to the direction of flow of the circulated drying gas in the treatment chamber.
It is known from DE 19522028 A1 to check the drying by means of individual wood moisture measuring points within each zone. This requires a plurality of measuring points which on the one hand require a high amount of investments and maintenance in practice and on the other hand are time-consuming in mounting them and finally lead to a far from inconsiderable risk concerning the correct assignment of the measuring points to the individual zones. These methods are hardly used in practice for such reasons and because the known wood moisture sensors supply especially imprecise measurement results in the region of high wood humidities.
A connection that can be formulated mathematically between the respective moisture content and the drying rate can only be obtained in wood, and such of coniferous trees in particular, below the fiber saturation point. The fiber saturation point represents the moisture content below which no free water is any longer contained in the wood. Since above the fiber saturation point the drying rate is substantially only dependent on the supplied heat quantity, but not on the moisture content, a moisture balance can only be produced in a moisture region between the fiber saturation point and the desired final moisture according to a connection that can be formulated mathematically, i.e. in a comparably low moisture range of 12 to 25%, assuming an average fiber saturation point of 30 to 35% and a final moisture of 10 to 18%. The usual wood humidities at the beginning of the drying process lie in the range of between 50% and 150%. The differences in the moisture content that still exist after the drying above the fiber saturation point would thus have to be compensated in relatively short intervals, which would lead to extremely small drying rates and, consequently, small temporal temperature differences with the disadvantage of relatively high measurement errors.
The invention is therefore based on the object of providing a method of the kind mentioned above in such a way that a substantial moisture balance can be ensured in comparatively short time intervals, namely by maintaining a predetermined final moisture. Moreover, an apparatus is to be created with which it is possible to stack wood of different initial moisture without prior sorting into a treatment chamber and still achieve an even mean final moisture of the wood batch.
This object is achieved in accordance with the invention in such a way that the stacks of wood are dried in the zone with the on average highest wood moisture with a permissible maximum speed up to the fiber saturation point and the other zones are dried within the terms of achieving the fiber saturation point with different speeds in the time interval as predetermined by the zone with most moisture before the stacks of wood are dried in an even circulatory flow to the predetermined final moisture, and that the average moisture of the wood used for determining the drying speeds is determined in a heating phase from the heat quantity supplied to the wood.
A simple method is created by the invention which allows balancing average differences in moisture in the individual zones above the fiber saturation point, i.e. at a time at which there is still an at least approximately linear drying behavior of the wood. The zone in which the on average largest wood moisture is present is dried with a still permissible speed up to the fiber saturation point, at which speed there is still no damage to the wood. The drying speed in the other zones is reduced relatively to this in such a way that each zone achieves the fiber saturation point at approximately the same time. According to the invention it is relevant that the mean differences in moisture in the individual zones are balanced at the latest when the fiber saturation point has been reached. Below the fiber saturation point the stacks of wood of all zones are dried in a uniform circulatory flow to the predetermined final moisture.
In order to determine the differences of the average wood humidities between the individual zones as precisely as possible, the mean moisture of the wood is determined in a heat-up phase from the heat quantity supplied to the wood. This heat quantity is composed in the known manner from the oven-dry weight (wood at 0% moisture) and the heat quantity supplied to the water contained in the wood. With the knowledge of the quantity of wood as introduced into the treatment chamber and the heat quantity supplied to the wood it is possible to determine the moisture content of the wood. It is especially advantageous when the mean moisture of the wood is determined in zones during the heat-up phase of the drying gas guided in a circulatory fashion from the heat quantity that is supplied to the wood during a monitoring period.
The determination of the heat quantity supplied to the wood occurs for example from the product of the temperature difference between the entrance of the drying gas into and the exit out of the stack with the mass of the circulated drying gas and a material constant of the drying gas. It is thus merely necessary to acquire the temperature difference between stack entrance and stack exit with at least one temperature sensor each, thus leading to a cost-effective, precise method for determining the supplied heat quantity. In order to determine the heat quantity supplied to the wood even more precisely, the heat quantity which is emitted to the drying gas by a heat exchanger and supplied to the wood is further determined from the product of the temperature difference between advance flow temperature and return flow temperature of the heat exchanger with the mass of the heating medium circulated per unit of time in the heat exchanger and a material constant of the heating medium. In order to calculate the heat quantity supplied to the wood, it is possible to use either the one or the other method. It is advisable to always use the mean value of the two methods for calculating the moisture of the wood. In addition, the wood temperature or the stack exit temperature of the drying gas can be measured during the heat-up phase at at least one point per zone and supplied to a memory unit. The mass of the water contained in the wood can then be determined from the heat quantity supplied during the monitoring period to the wood and the wood temperature difference and/or the drying gas temperature difference between the beginning and end of the monitoring period with knowledge of the volume of wood to be dried. If an especially precise determination of the wood moisture is required, the temperature of the drying gas at entrance into and exit from the stack, the temperature difference between advance flow temperature and return flow temperature of the heat exchanger and the wood temperature difference between a start and end of the monitoring period are considered for calculating the heat quantity supplied to the wood.
Without any additional wood measuring points the new method allows determining the initial moisture distribution between the individual zones of a treatment chamber. The initial moisture content can be determined according to the new method from the heat quantity supplied to the wood during the heat-up phase and the associated temperature change of the wood. The relevant aspect for the method in accordance with the invention is that it is not the determination of a precise absolute moisture content of the wood of the individual zones is relevant for a precise final result of the drying process. Instead, merely the relative differences between the individual zones need to be determined. It is an additional advantage that the determination of the differences between the individual zones occurs in the state of maximum temperature differences between stack entrance and exit, thus producing the best possible measurement precision. In accordance with the invention, drying is started after the termination of the heat-up phase and the drying course is guided based on the most humid zone according to a drying schedule as chosen by the operator. For this purpose, the associated heating devices are regulated preferably in all zones to a predetermined stack exit temperature of the drying gas. In order to enable the quickest and simplest possible discharge of the steam quantity issued to the drying gas from the zone or the treatment chamber or in order to enable maintaining the drying speed, it is proposed in accordance with the invention that the drying speed to be maintained for each zone during the drying phase is controlled by the quantity of drying gas replaced by fresh gas. Within the terms of the desired even final moisture in all zones, a drying gas quantity is exchanged which differs in each zone from the determined initial moisture differences. In this process, the stack exit temperature of the drying gas is regulated in all zones either to a uniform value or the same quantity of drying gas is exchanged in each zone per unit of time and the stack exit temperature of the drying gas is controlled in all zones depending on the steam quantity to be discharged.
In order to obtain a lower scattering of the mean final moisture it is proposed that individual zones are subdivided into sub-zones in which the stack exit temperature of the drying gas is controlled depending on the drying speed to be maintained.
Apparatuses for drying stacked wood in a drying gas comprise a treatment chamber for example which is subdivided into at least two zones for receiving wood as well as a control unit. The drying gas for each zone is circulated by at least one fan over at least one heat exchanger and a temperature sensor. In order to ensure that the stacks of wood of a zone with the highest average wood moisture can be dried up to the fiber saturation point and the other zones can be dried in the same time interval with a respective lower speed to the fiber saturation point, at least one feed line and one discharge line for the exchange of the drying gas is provided. The exchange of air enriched with steam can occur in a simple fashion through said feed and discharge lines without influencing adjacent zones. In order to enable the regulation of the exchanged drying gas, the feed and/or discharge line is associated with a control valve and/or a blower. The control of the valves or the output of the blower is made in such a way that a certain basic position is predetermined depending on the measured values. This basic position can be valid for all valves or blowers of a zone and is overlapped by corrective values which are determined from the initial moisture. In this way a zone with low initial moisture will dry to a slower extent precisely by such an amount which is necessary in order to achieve the fiber saturation point at the same time as the zone with humid wood. Since the exchanged air quantity in the individual zones is precisely in proportion to the respectively required drying performance, the virtually same absolute drying gas moisture contents are obtained in all zones for physical reasons. As a result, a single sensor is sufficient for detecting the absolute air moisture per treatment chamber.
Alternatively, and especially in additionally defined sub-zones, the drying speed (=drying rate) can also be varied in such a way that the thermal output is varied via the detour of a value determined differently on purpose for the exit temperature of the air. The fact is made use of that under given conditions and fresh gas exchange rates the speeds in the individual zones depend in a manner that can be mathematically formulated on the difference of the exit temperatures, with decreasing exit temperatures leading to decreasing drying rates.
In order to regulate the heat quantity supplied per zone, each heat exchanger comprises at least one valve for throttling or blocking its heating medium. In order to prevent any mutual influencing of the partial streams of the drying gas which are circulated in the individual zones, the fan(s) and the heat exchanger form a heating device in each zone which are preferably separated preferably by means of guiding devices for the drying gas from the heating devices of other zones and at least partly by means of a inserted ceiling and/or a separating wall from the treatment chamber. A zone is created which is cut off on the one hand from the other zones and on the other hand from the treatment chamber, in which zone an exchange of the drying gas occurs and in which heat is supplied to the drying gas and the drying gas is circulated through the zone. In order to reduce the influencing of the individual zones among each other even further, the treatment chamber can be provided with separating devices between the individual zones. These could be insertable separating walls, rolling doors, louvers or the like.
If the moisture differences of the wood are to be even better compensated within the individual zones, it is necessary to subdivide the individual zones into sub-zones for which separate drying gas conduits are arranged. As a result, at the beginning of the drying process it is necessary to determine the moisture content of the wood in every single sub-zone and then it is necessary to dry every single sub-zone per se at a respective speed. For this purpose, heating registers are provided which are associated with the sub-zones, can be triggered or regulated separately from each other and regulate the drying speeds in the individual sub-zones through the heating gas temperature.